It is often the case that a load such as an electric motor is driven by an output current in accord with the polarity and the magnitude of a differential input voltage. An example of such motor is a single-phase electric motor for driving a fan, a voice coil electric motor (VCM) of a hard disk drive (HDD), and a dc motor for driving a VTR, a CD-ROM, and a DVD drive.
Some of them are controlled by a signal or signals controlling on-off operation of the output transistors of the drive circuit of the motor based on the comparison of the input voltage with a reference voltage. This type of drive circuit, however, has a drawback in that the output current sharply changes across the point where the polarity of the output current changes (or zero-crossing point), and generates big noise. Furthermore, an inflow transistor circuit and an outflow transistor circuit of the drive circuit can be simultaneously turned on to allow a so-called huge penetration current to flow, and hence requires a delay circuit to circumvent the penetration current.
In order to circumvent such sharp change in the output current by smoothly switching the polarity of the output current across the zero-crossing point, an operational amplifier may be used to control the output current of the power amplifier circuit of a drive circuit. (See, for example literature 1; Masaomi Suzuki, “Standard Textbook: Designing Transistor Circuits”, 13th edition, CQ Publishing Co., Jul. 1, 1998, p. 315, FIG. 27.)
However, a drive circuit utilizing an operational amplifier not only has a complicated circuit configuration but also requires extra phase compensation capacitors. Since the drive circuit is usually built in on an IC chip, the chip must have a large area for the phase compensation capacitors. This raises the production cost of the drive circuit.